As fixing devices used in image forming apparatuses of an electrophotographic printing method for instance copying machines and printers, fixing devices of a heat roller fusing method have been frequently used. The fixing device of the heat roller fusing method includes: a pair of rollers (fuser roller and pressure roller) that are pressured against each other; and heating means made up of a halogen heater or the like, provided inside either both or one of the pair of rollers. After the heating means heats the pair of rollers to a set temperature (fixing temperature), a sheet (recording paper, recording sheet) on which an unfixed toner image is formed is fed to a pressure area (fixing nip area) of the pair of rollers; by causing the sheet to pass through the pressure area, the toner image is fused by the heat and pressure.
It is generally the case that in a fixing device provided in a color image forming apparatus, an elastic roller is used, which elastic roller is a fuser roller on which an elastic layer made of silicon rubber or the like is provided as its outermost layer. By having the fuser roller be an elastic roller, the surface of the fuser roller elastically changes in shape in accordance with an uneven surface of an unfixed toner image that comes in contact with the toner image, so as to cover up the toner image surface. This allows fusing, in a good manner, of a colored unfixed toner image which is greater in toner amount as compared to a black-and-white unfixed toner image.
Moreover, due to a warp-releasing effect of the elastic layer at the fixing nip area, it is possible to improve releasability of color toner that becomes more easily offset as compared to black-and-white toner. Furthermore, the nipping part in the fixing nip area has its top side (the fuser roller) protruding (what is called, reverse-nip shape), thereby improving the releasing property of the sheet. As a result, it is possible to release the sheet without using releasing means such as a releasing nail or the like (self-stripping). This overcomes the generation of an image defect caused by the releasing means.
In order to correspond to the speeding up of processing speed, it is necessary for the fixing device provided in such a color image forming apparatus to have a wide nip width in the fixing nip area. Examples of means for widening the nip width encompass: a method of thickening the elastic layer of the fuser roller; and a method of broadening a diameter of the fuser roller.
However, since the thermal conductivity of the elastic layer is extremely low, if heating means is provided as in the conventional fuser roller, inside the fuser roller which has the thickened elastic layer, this would result in insufficient heat supply in the case where the processing speed is sped up. As a result, a problem arises that the temperature of the fuser roller cannot keep up with the processing speed. On the other hand, if the diameter of the fuser roller is broadened, problems arise such as that more time is necessary to warm up the fuser roller and that the electricity consumption increases in amount.
In order to solve these problems, a fixing device of a belt fixing method (belt fixing device) such as one disclosed in Patent Literature 1 for example has been used in recent years as a fixing device provided in the color image forming apparatuses. The belt fixing device provides a heat roller externally of the fixing roller and provides a fixing belt across the fuser roller and the heat roller; the fuser roller and pressure roller are pressured against each other with the fixing belt being sandwiched between the fuser roller and the pressure roller.
The belt fixing device heats a fixing belt that has a small heat capacity, so it is therefore possible to shorten the time required for warming up. Moreover, no heat source such as a halogen lamp needs to be built in the fuser roller; this allows providing an elastic layer of a low hardness made of sponge rubber or the like, thereby making it possible to have a broad nip width.